Hi prpa,
Do you have any fins beyond the stock fin (or did your board come without a fin (2010 iSonic boards do not))?
What sort of windspeeds and water conditions are you going to be sailing in/on?
What is your min. wind threshold (are you wanting to be the earliest 100 Kg.
planer in your area)?
At 100 Kg. you are possibly limiting your early planing threshold with a max. 9.5 m2
RSS race sail.
It would seem a 10.0-11.0 free race (V8 maybe?) would get you planing earlier if that's
what you are after.
As far as fin size, at your size, it would seem a 60-66 cm on a 93.5 cm would get you planing the earliest. (Maybe even up to a 70 cm???)
If you are after the best speed, 52-60 cm would be the range I would suggest, but I do not weigh 100 Kg. so I may not be the best guy to ask.
Fin selection is going to be dependent on whether you want the earliest planing, or the best speed..... two different goals, that each require a different sort of fin.
A big powerful fin/foil to get you planing early and keep you planing when the wind drops below your threshold.
or
A slimmer slippery smaller fin/foil that has the least drag to give you max. speed.
Hope this helps,

Hi prpa,
I hope you have a smaller board or 2 to cover about 18 knots and up.....
The Isonic 150 in 20-28 knots of wind would be very much like a formula
board and probably quite a handful to sail.
The extreme range I suggested was to help you get the iS 150 going in < 12 knots of wind (fin range in the 65-70 cm).
With a huge fin, and really good pumping skills you can often lower the min. planing threshold a couple of knots windspeed, but it's alot of work, and the board is not real fast on the top end due to far too much drag from the fin.
But...... you are planing when other sailors your size are not!
For normal slalom type sailing the 56 cm stock fin would be OK for the mid range (say 12 knots to 16 knots with your 9.5).... I'm guessing you switch down to your 7.8 m2 race sail in around 17 knots windspeed....right?
So, perhaps a good aftermarket 58-62 cm for normal early planing, a 54-58 cm (or use the stock 56 cm) for the mid range, and a smaller 48-52 cm for higher winds and your smaller sail.
I know fins cost alot, but you never have enough of them.
Sailing a board/fin/sail combination that are perfectly balanced for the conditions is like a dream. Doesn't happen all that often, but when it does everything works; your speeds are very high; and you can't miss a jibe if you try.
hope this helps,